"Couldn't believe it at all, but I think that's what we're built for,'' Tatum said.
They might be built for a championship.
Tatum scored 29 points, Jaylen Brown had 22 and the Celtics completed a four-game sweep of the Brooklyn Nets, becoming the first team to reach the second round of the playoffs with a 116-112 victory Monday night.
Marcus Smart added 20 points and 11 assists for the No. 2-seeded Celtics, who made easy work of what was thought could be a tough series. Boston will wait for the winner of the series between Milwaukee and Chicago, in which the defending champion Bucks hold a 3-1 lead.
The Celtics showed no fear of the Nets, who with Kevin Durant and Kyrie Irving didn't have the profile of the typical low seed. Boston won on the final day of the regular season instead of losing to fall down the standings and avoid the matchup.
Turns out, there was no reason to worry.
"It's funny to us, because we don't duck and dodge anybody,'' Smart said. "We knew what we were going to come in here and do.''
Durant had 39 points, 9 assists and 7 rebounds, breaking free after the Celtics had defended him well in the first three games. But he couldn't get the Nets the win they needed to extend what instead goes down as a massive failure of a season.
Expected to be an NBA Finals contender, the Nets needed a strong finish just to grab the No. 7 seed.
Seth Curry scored 23 points for the Nets. Irving, whose unavailability for home games much of the season because he isn't vaccinated against the coronavirus contributed to the Nets' performing so far below expectations, had 20 points.
"A lot of stuff that may factor into why we lose, but they were just a better team,'' Durant said.
The Celtics were under .500 at the midpoint of their season but went 31-10 from there, with Tatum emerging as the potent scorer on one of the NBA's best defensive teams.
That defense limited Durant to just 19-for-52 shooting with 17 turnovers through the first three games and made it tough for him, even as he finally got more shots to drop Monday. He was only 13-of-31 from the field, 3-for-11 behind the arc.
He lost in the first round for the first time since 2010 with Oklahoma City, the first playoff series of his career. He had won nine straight first-round series.
Tatum had 12 points in the third quarter and the Celtics opened a 90-75 lead in the closing seconds of the period. But he went to the bench with a fifth foul with 8:17 remaining and the Nets made a push that got them within three points about three minutes later.
Tatum fouled out with the Celtics leading by six with 2:48 to play. Irving made a 3-pointer and Durant scored to cut it to 109-108 with 1:28 remaining. But after a basket by Brown, Durant could make only one free throw and the Celtics rushed down the floor, with Al Horford putting back Smart's miss to make it 113-109 with 13 seconds left.
The Nets were down only 45-44 with 3:29 to play in the second quarter. But Grant Williams made three 3-pointers from then until the end of the half, with the final one giving Boston a 58-50 lead.
The Associated Press contributed to this report.